Generally, when measuring the weight of the cooking stuff in a microwave oven, a piezo-electric sensor is used.
The piezo-electric sensor is made of a ceramic material which is capable of converting a mechanical energy to an electric energy, and vice versa. In this weight sensor, electric charges are generated proportionately to the applied pressure, which is defined by Q=.DELTA.Pxd, where .DELTA.P represents the variation of pressure, d the piezo-electric constant, and Q the amount of charges.
FIG. 1 illustrates the structure of the piezo-electric sensor for detecting the weight of the cooking stuff in a microwave oven. A motor 12 is installed in such a manner that a rectangular driving shaft 13 of the motor 12 should be projected above a bottom 11 of a microwave oven 10. Further, a tray 17 is mounted on the rectangular driving shaft 13 by fitting the tip of the rectangular shaft 13 into a rectangular slot 18 which is formed at the center of the tray 17.
Between the tray 17 and the bottom 11, there is installed an annular support 15 having rollers 16, and a piezo-electric sensor 14 is installed on the track along which the rollers 15 of the annular support 15 move.
Thus, with a cooking stuff placed on the tray 17, if the cooking or unfreezing mode is selected to activate the microwave oven 10, the tray 17 is rotated by the driving motor 12. Together with the tray 17 which is rotated by the motor shaft 13, the rollers 16 of the annular support 15 make rolling movements, with the result that the piezo-electric sensor 14 is pressed down, and that the weight of the cooking stuff is sensed by the piezo-electric sensor 14 in the form of a charge component.
Under this condition, the charge Q which is generated by the variation of the pressure is defined as follows: EQU .DELTA.P=k.times.(m+M).times.g.times.sin.THETA.
where m represents the mass of the tray, M the mass of the cooking stuff, g the gravitational acceleration, and k a constant.
Accordingly, the generated charges are as follows: EQU Q=k'.times.(m+M).times.g.times.sin.THETA..times.d
where k' represents a constant.
Thus M and Q come to have a proportional relationship, and therefore, the weight can be measured by detecting the amount of charges.
However, an A/D converter of a microprocessor which is used in a microwave oven has a limit in the measurable voltage. Therefore, there is required a dual-sensor circuit in which the amount of charges generated by the piezo-electric sensor is converted to a voltage so as for the microprocessor to recognize it, and in the case of an overload, the generation of an over voltage has to be suppressed to protect the microprocessor.
FIG. 2 illustrates a weight sensing signal processing circuit. As shown in this drawing, the amount of charges which is sensed by the piezo-electric sensor 14 is supplied to a filtering section 20 which consists of resistors R1-R2 and a capacitor C1.
The sinusoidal waves which are generated by the piezo-electric device are liable to be deformed by the vibrations of the structure. In accordance with the capacity of the piezo-electric device, i.e., based on Q=V.times.C (where C is the capacity), a high voltage difference of over several scores of volts is resulted. Therefore, the filtering section 20 modulates the deformed waves into the normal sinusoidal waves, and lowers the variation value of the voltage to several volts so that the circuit should be able to easily perform controls. The weight sensing signals which have passed through the filtering section 20 are inputted into an amplifying section 21 which consists of resistors R3 and R4 and an operational amplifier OP1.
The amplifying section 21 amplifies the signals which have been filtered and stepped down at the prior stage. Further, the amplifying section 21 adjusts the gradient between the mass of the cooking stuff and the output voltage (input voltage of the microprocessor). Under this condition, the adjustment is made such that the microprocessor should output its highest recognizable voltage at the highest measurable weight.
The output of the amplifying section 21 is inputted into a rectifying section 22 which includes a diode D1 and a capacitor C1.
The signals which have passed through the amplifying terminal become a sinusoidal wave having a period corresponding to the revolution speed of the rollers. Thus, if the microprocessor is capable of recognizing the magnitude of this sinusoidal wave, and is capable of converting it to a weight value, then no more processing is required. However, it is a technically difficult to make the-microprocessor recognize the sinusoidal wave, and therefore, the complicatedness of the processing circuit is accompanied.
The rectifying section 22 adopts a hold circuit which divides the sinusoidal wave into half waves across the ground (0 V), and charges them again (integrate) to convert them into a dc voltage. Thus, the A/D converter of the microprocessor is made to recognize not a sinusoidal wave but a dc voltage, thereby converting it into a weight value.
Here, the diode D1 not only divides the sinusoidal wave into half waves, but also prevents the reverse flow of the integrated charges to the capacitor C2.
The output of the rectifying section is inputted into a buffer section 23 which includes an operational amplifier OP2 and a zener diode ZD1.
The charges which are charged at the prior stage is converted into a dc voltage based on the relation V=Q/C. Although it is a very short period of time, during the time when the sinusoidal waves are charged, the voltage is not stabilized. Therefore, in order to achieve a more precise measuring of the weight, it is desirable that the voltage is read after a certain period of time (about 0.1 seconds) from the time of the generation of the sinusoidal waves to the time of converting it into the weight value. This is for taking into account the charging time of the sinusoidal wave. However, there is another problem, that is, the charges are discharged due to the impedance between the two electrodes of the capacitor C2, and this discharge causes a dissipation of the voltage in a log scale.
The buffer section 23 is installed for inhibiting the dissipation of the charges due to the low impedance of the input terminal of the microprocessor. The higher the input impedance of the buffer section 23 is, the larger the impedance between the two electrodes of the capacitor C2 is. Therefore the voltage step-down is reduced, so that the errors can be reduced during the weight measurement. Meanwhile, the zener diode ZD1 protects the microprocessor by preventing the input of an over-voltage into the microprocessor during the over-loading of the cooking stuff. A reset section 24 which includes a transistor Q1 supplies reset signals to the rectifying section 22.
The rollers of the microwave oven is provided in a plurality (usually 3) for the sake of the mechanical stability. The weight of the cooking stuff is distributed to the rollers during the revolutions. Therefore, in order to carry out a precise measuring, it is desirable that the measurements are carried out as many times as the number of the rollers, and the weight is calculated out by taking the average weight.
In carrying out the measurements many times, the reset section 24 discharges the capacitor C2 of the rectifying section 22 each time the measurement is made, and thus the reset section 24 serves as a switch.
However, in such a conventional circuit, the microprocessor cannot fully utilize the recognizable range in the proportional relation between the mass of the cooking stuff and the output voltage due to the existence of the mass of the tray as shown in FIG. 3. The reason is that the origins (0,0) of the voltage V and the mass of the cooking stuff g do not correspond with each other due to the influence of the mass of the tray.
For example, it is assumed that the A/D converter of the microprocessor recognizes 255 (2.sup.8 -1) steps, and, with this 255 steps, have a proportional decimal range of recognizing 0-5 V. Then the relation between the voltage and the weight of the cooking stuff in relation with the mass of the tray is as shown in Table 1 below.
TABLE 1 __________________________________________________________________________ Conventional Relation between voltage and weight of cooking stuff Mass of food Vltg diffntl/g Mass of tray 0 g 1500 g Utlzbl range Vmax - Vmin (m) Dec Vltg Dec Vltg Vmin Vmax 1500 __________________________________________________________________________ Model 1 800 g 180 -1.471V 0 -5V -1.471V -5V -0.0023527V Model 2 1200 g 160 -1.863V 0 -5V -1.863V -5V -0.0020913V __________________________________________________________________________ Remarks: *It is designed that a weight of the cooking stuff of 0-1500 g can be measured. *By using 8bit data, a division is made into 255 steps (2.sup.8 -1) from the maximum voltage value to the minimum voltage value. *A recognizable voltage range of 0-5V is assumed.
Thus in order to improve the precision of the measurement of the weight of the cooking stuff, the voltage differential per g has to be improved, so that the microprocessor should be able to recognize even a small variation of the weight. In order to improve such performance, the influence of the tray which affects the proportional relation of the voltage and weight has to be minimized.
Theoretically, in the mass value of the tray in the conventional circuit, if the weight of the tray is taken to be 0 g, then its influence is completely eliminated (because 0 times a real number is 0), and therefore, the voltage differential per g can be enlarged by adjusting the amplification ratio of the amplifying section 21. Practically, however, cooking without a container is inconceivable in a microwave oven, and therefore, a mechanical improvement cannot be expected. Further, the influence of the mass of the tray is increased proportionately to the capacity of the tray, and therefore, this factor constitutes an impediment in enlarging the capacity of the microwave oven.